Republican candidates stepped up their game for a last-minute push to attract undecided Republican voters just hours away from the nation's first nomination vote, the Iowa caucuses.

The campaigns also fixed their sights on the next big contests that will take place the rest of this month.

Front-runner Mitt Romney's campaign today released a TV ad in Florida, which will hold its presidential primary Jan. 31. The ad, titled "Leader," has run in Iowa, New Hampshire and is making the rounds on air in South Carolina, his next big test.

In a sign that Romney might be sensing victory, the former Massachusetts governor has shifted his focus away from assailing his rivals -- namely Newt Gingrich -- and onto President Obama.

"This has been a failed presidency," Romney said at an event in Des Moines today. "I will go to work to get Americans back to work."

At the Romney campaign headquarters in Iowa today, volunteers, armed with scripts, made phone call after phone call urging supporters and undecided Republicans to head to the caucuses and deliver a victory for him.

Romney made an aggressive push in Iowa in the seven days leading up to the caucus, covering more ground in the state than he has during the rest of the primary season combined.

Nearly four out of ten Iowa Republican voters were undecided, according to a Des Moines Register poll, giving the candidates leeway to campaign late into the evening.

Clive resident John Brown's decision to support Romney came into focus only in the last few days.

"Mitt Romney is the best suited to get our economy back on track and, come November, is really the best suited to defeat Barack Obama," Brown told ABC News on caucus eve as he attended a Romney rally with his young daughter. "My wife has lost two jobs since Obama became president, we've got to find some ways to get the economy going again, get the private sector going again so that people can get back to work."

The on-the-ground effort in Iowa was in full effect on caucus day, with several hundred calls being made across the state. New technology allowed some volunteers to make calls from their home computers.

Romney regained his top spot in the polls largely because of an aggressive campaign of negative attacks against Gingrich. Outside groups spent a total of $4.6 million on ads focused on the former speaker, who had surged in polls as others fell.

Gingrich, who said Monday he regretted not going on the attack earlier, blasted Romney today as a "liar" and challenged him to "just level with the American people" about his moderate views.

"This is a man whose staff created the PAC and his millionaire friends fund the PAC and it's baloney," the former House speaker railed today on CBS' "Early Show." "He's not telling the American people the truth. Here's a Massachusetts moderate who has tax-paid abortions in 'Romneycare' and puts Planned Parenthood in 'Romneycare' ... and appoints liberal judges to appease Democrats and wants the rest of us to believe he's somehow magically a conservative."

Romney only responded by saying he doesn't know why Gingrich "is so angry."

Super PACs, or outside spending groups, poured nearly $13 million in Iowa and other early-voting states until the start of 2012, according to an analysis by the Center for Public Integrity. Romney benefited most from this money, mainly from Restore Our Future, the group set up exclusively to help him. Texas Gov. Rick Perry came in second, receiving $3.7 million worth of help from outside groups.

Rick Santorum, meanwhile, basked in his newfound popularity. The former Pennsylvania congressman surged in the Iowa polls just days before the election, and is expected to attract a sizeable chunk of the influential evangelical vote in Iowa.

"This is the first step, this is the first step," Santorum told ABC News after his last Iowa campaign stop in Urbandale. "A win here is a start, but it's a start. It's not the culmination, it's a start. It's not the culmination, it's the beginning."

He was quoted by several news outlets as saying, "I don't want to make black people's lives better by giving them somebody else's money. I want to give them the opportunity to go out and earn the money and provide for themselves and their families."

Santorum was addressing entitlements at a town hall in Iowa, as he does at almost every campaign event, but it's unclear whether the GOP candidate actually said "black" people or simply stumbled on his words.